More often than at any point in recent memory, people have been getting away with murder in Philadelphia.

The city’s homicide clearance rate in 2016 dropped below 50 percent -- the lowest the city has seen in at least 15 years, and the third consecutive year that the rate has decreased, according to police statistics.

While the Homicide Unit posted a clearance rate above 70 percent as recently as 2012 and 2013 -- nearly 10 points higher than the nationalaverage -- last year, when there were 277 murders, the rate was just 45.4 percent, meaning police arrested dozens fewer murder suspects than they had just a few years earlier.

Theories for the downturn vary, from a slowly shrinking pool of homicide detectives, to a belief that media coverage of allegations of police brutality has fueled distrust in minority communities, worsening the decades-old challenge of finding cooperating witnesses.

Many within the Homicide Unit, however, also blame new interrogation policies implemented three years ago, which allow witnesses to decline interviews or leave them whenever they want. The rules, designed to protect the civil rights of witnesses and suspects -- and prevent police from eliciting false confessions -- also mandate that suspect interviews be recorded on video.

“They changed everything,” said one veteran investigator, who requested anonymity to speak frankly about the directives.

“Witnesses are a thing of the past,” said another.

A third described the rules with one word: “Crippling.”

Police Commissioner Richard Ross, a former commander of the Homicide Unit, was instrumental in crafting the new directives and has flatly said they are not going away.

Homicide Capt. James Clark said in an interview last week, “We just have to find other ways to get the end result done.”

Criminologists note that such policies are common in other cities with high clearance rates, and simply force detectives to build cases on surveillance video, cellphone records, or other forensic evidence, rather than relying on confessions or witness testimony.

While Ross has said he does not believe the directives have made solving cases too difficult, he has had trouble offering another theory behind the rate's drop.

“We have some excellent investigators. ... I’d put them up against any Homicide Unit in the country,” he said in an interview last year. “But I cannot explain this precipitous drop in the clearance rate.”

The effects of unsolved murders are felt by dozens of families each year. Depression, anger, and a recurring sense of dread are common, family members say, all of which compound the grief of unexpectedly losing a loved one to violence.

“We don’t know who to trust,” said Trina Singleton, whose 24-year-old son Darryl Singleton was shot dead behind the family’s Southwest Philadelphia home in September.

Details in the aftermath of Darryl Singleton’s killing -- one of three homicides and six shootings on Sept. 13 -- were scant. He was shot on the 5800 block of Hoffman Street around 7:36 p.m. and declared dead at Penn Presbyterian Medical Center less than 10 minutes later. Police reported no suspects or motive.

Darryl Singleton, who was studying to become an EMT, had been doing homework with his 5-year-old brother earlier that afternoon, his parents said. He was a day shy of his 25th birthday when he was killed.

His parents have since handed out fliers about the crime in the neighborhood, talked to friends, written to public officials, and encouraged anyone who knows anything to talk to police. Rumors have been swirling for months, the Singletons said in an interview last week, but the couple said they've been told by police that more is needed to secure an arrest.

In the meantime, the Singletons have felt jilted by their neighborhood -- and alone with their grief.

“There’s too many people who know,” Isaac Singleton said. “For a family that grew up on that block, you think there’d be more people to say, ‘Not him.’ ”

Difficulty convincing witnesses to come forward is not new.

For decades, authorities in Philadelphia and other cities have battled a so-called “no-snitch” culture. They’ve developed carrots, such as the city’s $20,000 reward for tips that lead to homicide convictions, and sticks, including promised punishment for those who attempt to intimidate witnesses in court. Just last week, the Criminal Justice Center announced that courthouse visitors will have to place their cellphones in locked pouches, an effort to prevent people from taking pictures of witnesses on the stand or other potential intimidation tactics.

Still, the phenomenon persists. And some experts subscribe to a theory common among police: that distrust between community members and law enforcement has grown in recent years, as the media has intensely covered the killings of unarmed black men by police in Ferguson, Mo., Chicago, North Charleston, S.C., and elsewhere.

“It’s like the boil has come to be festered to the point where it burst,” said Thomas Alexander, a criminal justice professor at the University of Maryland and a police lieutenant in Hagerstown, Md.

Alexander acknowledged that there isn’t much research yet to support the theory. But he pointed to Chicago as an example: As tensions rose and killings surged last year in the aftermath of a fatal police shooting, the clearance rate plunged. A Chicago radio station, WBEZ-FM, citing data obtained through a Freedom of Information Act request, reported in January that the rate in 2016 was just 19.8 percent.

A rise in gang-related killings also seemed to contribute to Chicago’s low clearance rate, Alexander said, since gang members are often the most difficult to convince to cooperate, and community members can be afraid of retaliation if they speak out.

“We need to rebuild that thing called police legitimacy,” he said. “If you see the police as being legitimate, a legitimate authority, you’re more likely to cooperate.”

Clark said high-profile cases often attract tips that can lead to an arrest, but that those murders are the exception rather than the rule. He also said he believed witness cooperation was becoming less frequent.

“A lot of the homicides we get in the neighborhoods, we need witnesses to come forward and tell us who did it,” he said. “And quite honestly, it’s a struggle sometimes.”

David L. Carter, a professor at Michigan State University who has studied clearance rates, said the community’s willingness to trust police is a vital factor. But he took issue with claims that the new interrogation rules in Philadelphia have stifled investigators.

Police departments from Denver to San Diego and Baltimore County, Md., have continued to clear homicides with the same requirements regarding witnesses and suspects, Carter said, and he described the regulations as “pretty standard” nationally. Homicide clearance rates also tend to be higher than any other crime, despite the obstacles.

Each of those jurisdictions typically records significantly fewer murders than Philadelphia, even though the city's recent homicide totals have remained low compared to previous decades.

Dennis Cogan, a prominent Philadelphia defense attorney, said in the absence of rules for witnesses and suspects, it would be much easier for investigators to cut legal corners by doing things such as physically abusing suspects to coerce confessions and holding witnesses against their will when they haven’t been charged with a crime.

He also said new technologies -- surveillance cameras, cellphone records, sophisticated DNA analysis -- have provided additional methods for police to build a case.

“It’s not like we’re in an era where the police can’t prevail,” said Cogan, who met with Ross and former Commissioner Charles H. Ramsey to advocate for such rules before they were implemented.

Clark said Ross has told him that the unit, which currently has 61 detectives, will receive a batch of new investigators within the next several weeks. Retirements had chipped away at the unit’s staff numbers in recent years, Clark said, but the impending influx -- which he declined to quantify -- should “bring [the unit] back up to proper staffing levels.”

Families like the Singletons would welcome any development that might help put their son’s killer behind bars.